Huanglongbing, HLB, or citrus greening disease was first reported in southern China in 1919. The disease is now found in approximately 40 different Asian, African, and North and South American countries and has recently become a serious threat in Florida, California, Louisiana, Texas, Cuba and Brazil, all of which are major citrus producing locations. Citrus greening disease is caused by the phloem-limited fastidious prokaryotic α-proteobacterium Candidatus Liberibacter spp., Ca. africanus, and Ca. L. americanus. 
Citrus trees that become infected with the citrus greening disease go into decline, producing misshapen, off-flavor fruit, and then die within a few years. The billion dollar (annual) Florida citrus industry is severely threatened by this vector-disease pathosystem. Further, the disease also threatens the citrus industry in California. There is, currently, no cure for this disease and trees are routinely destroyed once severely infected. Moreover, there are no known relevant cultivars that are resistant to citrus greening disease.